The problem of controlling static charge is well known in the field of photography. The accumulation of charge on film or paper surfaces leads to the attraction of dirt which can produce physical defects. The discharge of accumulated charge during or after the application of the sensitized emulsion layer(s) can produce irregular fog patterns or "static marks" in the emulsion. The static problems have been aggravated by increases in the sensitivity of new emulsions, increases in coating machine speeds, and increases in post-coating drying efficiency. The charge generated during the coating process may accumulate during winding and unwinding operations, during transport through the coating machines and during finishing operations such as slitting and spooling. Static charge can also be generated during the use of the finished photographic film product. In an automatic camera, the winding of roll film in and out of the film cartridge, especially in a low relative humidity environment, can result in static charging. Similarly, high speed automated film processing can result in static charge generation. Sheet films (e.g., x-ray films) are especially susceptible to static charging during removal from light-tight packaging.
It is generally known that electrostatic charge can be dissipated effectively by incorporating one or more electrically-conductive "antistatic" layers into the film structure. Antistatic layers can be applied to one or to both sides of the film base as subbing layers either beneath or on the side opposite to the light-sensitive silver halide emulsion layers. An antistatic layer can alternatively be applied as an outer coated layer either over the emulsion layers or on the side of the film base opposite to the emulsion layers or both. For some applications, the antistatic agent can be incorporated into the emulsion layers. Alternatively, the antistatic agent can be directly incorporated into the film base itself.
A wide variety of electrically-conductive materials can be incorporated into antistatic layers to produce a wide range of conductivity. These can be divided into two broad groups: (i) ionic conductors and (ii) electronic conductors. In ionic conductors charge is transferred by the bulk diffusion of charged species through an electrolyte. Here the resistivity of the antistatic layer is dependent on temperature and humidity. Antistatic layers containing simple inorganic salts, alkali metal salts of surfactants, ionic conductive polymers, polymeric electrolytes containing alkali metal salts, and colloidal metal oxide sols (stabilized by metal salts), described previously in patent literature, fall in this category. However, many of the inorganic salts, polymeric electrolytes, and low molecular weight surfactants used are water-soluble and are leached out of the antistatic layers during photographic processing, resulting in a loss of antistatic function. The conductivity of antistatic layers employing an electronic conductor depends on electronic mobility rather than ionic mobility and is independent of humidity. Antistatic layers which contain semiconductive metal halide salts, semiconductive metal oxide particles, etc., have been described previously. However, these antistatic layers typically contain a high volume percentage of electronically conducting materials which are often expensive and impart unfavorable physical characteristics, such as color or reduced transparency, increased brittleness and poor adhesion, to the antistatic layer.
Colloidal metal oxide sols which exhibit ionic conductivity when included in antistatic layers are often used in imaging elements. Typically, alkali metal salts or anionic surfactants are used to stabilize these sols. A thin antistatic layer consisting of a gelled network of colloidal metal oxide particles (e.g., silica, antimony pentoxide, alumina, titania, stannic oxide, zirconia) with an optional polymeric binder to improve adhesion to both the support and overlying emulsion layers has been disclosed in EP 250,154. An optional ambifunctional silane or titanate coupling agent can be added to the gelled network to improve adhesion to overlying emulsion layers (e.g., U.S. Pat. No. 5,204,219) along with an optional alkali metal orthosilicate to minimize loss of conductivity by the gelled network when it is overcoated with gelatin-containing layers (U.S. Pat. No. 5,236,818). Also, it has been pointed out that coatings containing colloidal metal oxides (e.g., antimony pentoxide, alumina, tin oxide, indium oxide) and colloidal silica with an organopolysiloxane binder afford enhanced abrasion resistance as well as provide antistatic function (U.S. Pat. Nos. 4,442,168 and 4,571,365).
Antistatic layers containing electronic conductors such as conjugated conducting polymers, conducting carbon particles, crystalline semiconductor particles, amorphous semiconductive fibrils, and continuous semiconducting thin films can be used more effectively than ionic conductors to dissipate static charge since their electrical conductivity is independent of relative humidity and only slightly influenced by ambient temperature. Of the various types of electronic conductors, electrically conducting metal-containing particles, such as semiconducting metal oxides, are particularly effective when dispersed in suitable polymeric film-forming binders in combination with polymeric non-film-forming particles as described in U.S. Pat. Nos. 5,340,676; 5,466,567; 5,700,623. Binary metal oxides doped with appropriate donor heteroatoms or containing oxygen deficiencies have been disclosed in prior art to be useful in antistatic layers for photographic elements, for example, U.S. Pat. Nos. 4,275,103; 4,416,963; 4,495,276; 4,394,441; 4,418,141; 4,431,764; 4,495,276; 4,571,361; 4,999,276; 5,122,445; 5,294,525; 5,382,494; 5,459,021; 5,484,694 and others. Suitable claimed conductive metal oxides include: zinc oxide, titania, tin oxide, alumina, indium oxide, silica, magnesia, zirconia, barium oxide, molybdenum trioxide, tungsten trioxide, and vanadium pentoxide. Preferred doped conductive metal oxide granular particles include antimony-doped tin oxide, fluorine-doped tin oxide, aluminum-doped zinc oxide, and niobium-doped titania. Additional preferred conductive ternary metal oxides disclosed in U.S. Pat. No. 5,368,995 include zinc antimonate and indium antimonate. Other conductive metal-containing granular particles including metal borides, carbides, nitrides and suicides have been disclosed in Japanese Kokai No. JP 04-055,492.
One serious deficiency of such granular electronic conductor materials is that, especially in the case of semiconductive metal-containing particles, the particles usually are highly colored which render them unsuitable for use in coated layers on many photographic supports, particularly at high dry weight coverage. This deficiency can be overcome by using composite conductive particles composed of a thin layer of conductive metal-containing particles deposited onto the surface of non-conducting transparent core particles whereby a lightly colored material with sufficient conductivity is obtained. For example, composite conductive particles composed of two dimensional networks of fine antimony-doped tin oxide crystallites in association with amorphous silica deposited on the surface of much larger, non-conducting metal oxide particles (e.g., silica, titania, etc.) and a method for their preparation are disclosed in U.S. Pat. Nos. 5,350,448; 5,585,037 and 5,628,932. Alternatively, metal-containing conductive materials, including composite conducting particles, with high aspect ratio can be used to obtain conducting coatings with lighter color due to reduced dry weight coverage (vide, for example, U.S. Pat. Nos. 4,880,703 and 5,273,822). However, there is difficulty in the preparation of conductive coatings containing composite conductive particles, especially the ones with high aspect ratio, since the dispersion of these particles in an aqueous vehicle using conventional wet milling dispersion techniques and traditional steel or ceramic milling media often result in wear of the thin conducting layer from the core particle and/or reduction of the aspect ratio. Fragile composite conductive particles often cannot be dispersed effectively because of limitations on milling intensity and duration dictated by the need to minimize degradation of the morphology and electrical properties as well as the introduction of attrition-related contamination from the dispersion process.
Moreover, these metal containing semiconductive particles, can be quite abrasive and cause premature damage to finishing tools, such as, knives, slitters, perforators, etc. and create undesirable dirt and debris which can adhere to the imaging element causing defects.
The requirements for antistatic layers in silver halide photographic films are especially demanding because of the stringent optical requirements. Other types of imaging elements such as photographic papers and thermal imaging elements also frequently require the use of an antistatic layer. However, the requirements there are somewhat different. For photographic paper, an additional criterion is the ability of the antistatic backing layer to receive printing (e.g., bar codes or other indicia containing useful information) typically administered by dot matrix or inkjet printers and to retain these prints or markings as the paper undergoes processing (viz., backmark retention). Yet another important criterion for photographic paper is its spliceability. Heat splicing of photographic paper rolls is often carried out during printing operations and is expected to provide sufficient mechanical strength to resist peeling as the web goes through automatic photographic processing. Heat splicing is typically carried out between the silver halide side of the paper and the antistatic backside of the paper. Poor splice strength can cause a number of problems including jamming of automatic processing equipment.
Electrically-conductive layers are also commonly used in imaging elements for purposes other than providing static protection. Thus, for example, in electrostatographic imaging it is well known to utilize imaging elements comprising a support, an electrically-conductive layer that serves as an electrode, and a photoconductive layer that serves as the image-forming layer. Electrically-conductive agents utilized as antistatic agents in photographic silver halide imaging elements are often also useful in the electrode layer of electrostatographic imaging elements.
An abrasion-resistant protective overcoat including a selected polyurethane binder, a lubricant, a matting agent, and a crosslinking agent overlying a conductive backing layer is described in U.S. Pat. No. 5,679,505 for motion picture print films; the abrasion-resistant protective overcoat contains a crosslinked polyurethane binder and, thus, provides a nonpermeable chemical barrier for antistatic layers containing, preferably, colloidal vanadium pentoxide antistatic agent which is known to degrade in contact with photographic processing solutions. Although U.S. Pat. No. 5,679,505 can provide certain advantages over conventional carbon black containing backing layers, the use of a crosslinking agent in the topcoat (without which the conductivity of the preferred antistatic layer will be jeopardized) poses some manufacturing concerns: crosslinked polyurethanes of U.S. Pat. No. 5,679,505 may impose additional constraints on the composition and pot-life of the coating solutions as well as other manufacturing parameters; from a health and safety standpoint, some crosslinking agents may require special handling and disposal procedures; removal of a crosslinked polyurethane layer can hinder recycling of the support.
As indicated above, the prior art on electrically-conductive layers in imaging elements is extensive and a very wide variety of different materials has been proposed for use as the electrically-conductive agent. There is still, however, a critical need in the art for improved electrically-conductive layers which are useful in a wide variety of imaging elements, which can be manufactured at reasonable cost, which are environmentally benign, which are durable and abrasion-resistant, which are effective at low coverage, which are adaptable to use with transparent imaging elements, which do not exhibit adverse sensitometric or photographic effects, and which maintain electrical conductivity even after coming in contact with processing solutions (since it has been observed in industry that loss of electrical conductivity after processing may increase dirt attraction to processed films which, when printed, may cause undesirable defects on the prints).
It is toward the objective of providing improved electrically-conductive layers that more effectively meet the diverse needs of imaging elements--especially of silver halide photographic films but also of a wide range of other imaging elements--than those of the prior art that the present invention is directed.
Use of smectite clay in imaging elements has been disclosed before. For example, in European Patent Application 0644454A1 use of synthetic smectite clay in an antistress layer for x-ray films has been disclosed. In U.S. Pat. No. 5,478,709 use of synthetic clay in the silver halide emulsion layer for reduction in roller marks during automatic processing has been described. The use of synthetic hectorite as an additive to a silica containing antistatic layer has been proposed before in U.S. Pat. Nos. 4,173,480 and 5,494,738. However, the integrity of these layers, when present as external layers, in contact with processing solutions during high speed processing is likely to be minimal resulting in loss of post-processing antistatic characteristic. In fact, some of the drawbacks of external antistatic layers containing a combination of hectorite clay and silica applied on photographic paper have been described in commonly assigned copending application Ser. No. 08/937,685. Although some binders have been mentioned in U.S. Pat. No. 4,173,480 for their use in conjunction with hectorite clay for application as a surface sizing agent for the fibrous paper base, the binders mentioned therein are hydrophilic binders such as gelatin, starch and methyl cellulose which are likely to offer little resistance to the processing solutions. The use of an organic compound which can intercalate inside and/or exfoliate smectite clay has been taught in commonly assigned copending applications U.S. Ser. Nos. 08/937,685 and 08/940,860 for application in antistatic layers containing smectite clay. The use of vinylidene halide based interpolymer binders for antistatic layers containing smectite clay has been taught in U.S. Ser. No. 08/992,857 such layers provide antiscumming properties as well as black and white process-surviving antistatic properties. However, none of these inventions can provide process-surviving antistatic characteristics after a typical color photographic processing.
Electrically conducting polymers have recently received attention from various industries because of their electronic conductivity. Some of these electrically conducting polymers, such as substituted or unsubstituted pyrrole-containing polymers (as mentioned in U.S. Pat. Nos. 5,665,498 and 5,674,654), substituted or unsubstituted thiophene-containing polymers (as mentioned in U.S. Pat. Nos. 5,300,575; 5,312,681; 5,354,613; 5,370,981; 5,372,924; 5,391,472; 5,403,467; 5,443,944; 5,575,898; 4,987,042 and 4,731,408) and substituted or unsubstituted aniline-containing polymers (as mentioned in U.S. Pat. Nos. 5,716,550 and 5,093,439) provide electronic conductivity instead of ionic conductivity, and, hence are conducting even at low humidity. Unlike metal-containing semiconducting particulate antistatic materials (e.g., antimony-doped tin oxide), the aforementioned electrically conducting polymers are less abrasive and environmentally more acceptable due to absence of heavy metals. However, one disadvantage of such polymers is their color which can prohibit their use in certain photographic applications.
Use of inorganic "powdery or granular" material coated with a conducting polymer has been disclosed in U.S. Pat. Nos. 4,937,060 and 4,956,441. However, the optical properties of the granular material and their suitability for application as transparent coatings for imaging application have not been addressed in these patents. Moreover, the processes described in U.S. Pat. Nos. 4,937,060 and 4,956,441 involved polymerization of the electrically conducting polymer in the presence of the granular material leading to a solid mass that needed to be separated and processed further for its end-use, unlike the present invention to be described in detail hereinbelow.
Japanese patent application JP2194071 A disclosed non-conductive pigment coated with conductive polymers for possible use as conductive primers for coating electrostatically non-conductive materials but required the use of an additional conductive filler such as metal powders, semiconducting metal oxide powder, carbon black, etc.
Japanese patent application JP 6167778 A disclosed antistatic films containing amorphous powder and an electroconductive polymer but did not teach of a layered siliceous material which is crystalline and can host an electrically conducting polymer through intercalation, as per the present invention. Moreover, given the ionic nature of the disclosed electroconductive polymers, it is unlikely that the antistatic films of JP 6167778 A would survive a color photographic processing.
As will be demonstrated hereinbelow, the present invention provides an auxiliary layer to an imaging element, with antistatic characteristics, before and after a typical color photographic processing, with or without a protective topcoat.